The haptoglobin genetic locus at 16q22 is polymorphic with two known classes of alleles denoted 1 and 2 (Langlois M et al, Clin Chem 42: 1589-1600, 1996). The polymorphism is quite common, with worldwide frequencies of the two alleles being approximately equal. Haptoglobin is a major susceptibility gene for the development of diabetic vascular complications in multiple longitudinal and cross-sectional population studies (Levy A et al, New Eng J Med 343: 969-70, 2000; Roguin A et al, Am J Cardiol 87: 330-2, 2001). Diabetic individuals homozygous for the haptoglobin 2 (Hp 2) allele are at 5 times greater risk of developing cardiovascular disease as compared to diabetic individuals homozygous for the haptoglobin 1 allele (Hp 1), with an intermediate risk present in the heterozygote (Levy A et al, J Am Coll Cardiol 40: 1984-90, 2002). The risk pertains to both type 1 and type 2 diabetes (Costecou, T. Ferrell, R E, Orchard T J. Haptoglobin genotype: a determinant of cardiovascular complication risk in Type 1 diabetes. Diabetes 57:1702-1706 (2008)). Mechanistic studies using the purified protein products of the Hp 1 and Hp 2 alleles have identified profound differences in antioxidant and immunomodulatory activity (Frank M et al, Blood; 98: 3693-8, 2001; Asleh R et al, Circ Res 92: 1193-200, 2003).
Functional as well as structural differences exist between the various haptoglobin allelic protein products (Langlois M et al, Clin Chem 42: 1589-1600, 1996). The Hp 2 allele has two copies of exon 3 and 4 of the Hp1 allele, which results in the duplication of a multimerization domain in exon 3. Consequently, while the Hp1 allele protein product forms only dimers, Hp2 allele protein products combine to form cyclic polymers ranging in size from three monomers and upwards. In heterozygotes, linear polymers containing both allelic protein products are present.
The development of an antibody based ELISA test to type haptoglobin has been hampered by the apparent lack of antigenic determinants unique to either allelic protein product. Apart from a single junction at the site of duplication of exon three, there exist no differences in primary amino acid sequence between the haptoglobin alleles. Given the need to screen large populations of diabetic individuals (10% of the western world) for their haptoglobin type in order to determine optimal treatment as well as the need to screen certain populations rapidly (i.e. individuals suffering from an acute myocardial infarction) there is a great need for a simple, rapid, inexpensive test for haptoglobin phenotyping.